Theme Lecture – Numerical Modeling of TLP – Foundation Interaction under Seismic Loading and Seabed Liquefaction
The presentation will focus upon the results of numerical analyses of TLP foundations during seismic loading and seabed liquefaction, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when liquefaction in the subsoil is extensive, leading to degradation of the pseudo static factors of safety against pullout failure of the pile well below 1.0. It is shown that the pile resistance to pullout failure decreases drastically during shaking, but fully recovers during the following dissipation of earthquake-induced excess pore pressures and even exceeds the initial (pre-shaking) resistance value. Pile head displacements develop steadily during shaking and the following dissipation phase, but only during the limited time period when the static pullout factor of safety of the pile remains less than unity. Final pile head displacements increased considerably when the seismic excitation was upgraded to the ALE level and the coefficient of soil permeability was decreased to that of fine silty sands. Due to the very high tensional stiffness of the tendons, relative to the buoyancy stiffness of the platform, the pile head pullout for shallow water depths (i.e. up to 100-200m) is almost directly transmitted to the platform, with only a very small part corresponding to reduction of tendon elongation and loss of pretension. However, the loss of pretension may increase up to 50-60% at deep water depths (i.e. above 1000m). It is also shown that, for the simplified single support TLP examined in the paper, the potential loss of buoy stability and tendon pretension may prove detrimental, but it is unlikely to threaten the safety and functional capacity of the platform. This finding needs to be further verified for more general design conditions, where the platform is stabilized with more (e.g. 3 or 4) sets of tendons while seismic shaking occurs simultaneously with other environmental loads (e.g. high sea waves and/or strong currents).
Key Words: TLPs, piles, seismic liquefaction